1. Field of the Invention
The present invention relates to a mobile communication system and a mobile station paging method, and more particularly, to a mobile communication system which comprises a plurality of base stations for paging a mobile station at which an incoming call is to be received, a location register for storing location information from a plurality of mobile stations, a mobile station paging signal transfer node interconnecting those base stations and the location register by physical or logical means; and to a mobile station paging method for sending a mobile station paging signal selectively to particular base stations in a location area where a mobile station to be paged is expected to be present.
2. Description of Related Art
A mobile station paging signal (hereafter referred to as a paging signal) is broadcast from a base station prior to establishing a communication link for an incoming call with a particular mobile station in a mobile communication system. If the mobile station at which the incoming call is to be received is present in a base station area covered by the radio signal of the base station, and if the mobile station is available to accept the incoming call, the mobile station responds to the paging signal. This is the basic procedure for identifying a mobile station with which a communication link is to be established.
A conventional paging method for a mobile station in a mobile communication system is described in "Public Land Mobile Network Interworking with ISDN and PSTN: CCITT Recommendations Q.1001 to Q.1032", Fascicle, VI.12, pps 5-16.
As described in this reference, the service area of a mobile communication system for mobile communication services consists of one or more location areas, and each location area consists of one or more base station areas. The base station area is defined as an area covered by the radio signal of one base station, and the location area is defined as that area in which a mobile station may move freely without updating its location information. The location information is used for identifying a location area in which a mobile station exists at present, and it is stored in a location register.
Therefore, when an incoming call is being sent to a mobile station, the location area where the mobile station is at that time is identified by the location information in the location register, and all base stations in the location area are instructed to broadcast a paging signal to locate the desired mobile station.
FIG. 1 is a block diagram showing the conventional mobile station paging method illustrated in the above-mentioned reference.
A mobile services switching center (MSC) 42 is connected to a transit switching center 41 for call connection with a public switched telephone network (PSTN), and to base stations 431 to 434 for connection with mobile stations (not shown) by radio signal. The area controlled by the MSC 42 is called a controlled area or an MSC area, and this MSC area consists of two location areas, the location area 1 and the location area 2 as an example in FIG. 1. In this example, each location area consists of base station areas covered by base stations 431, 432 and base stations 433, 434, respectively.
As the location area is that area in which a mobile station may move freely without updating its location information, once it has moved to the other location area, the mobile station should report its change to a database managed by the location register (not shown) and update the location information to a new location area which has been notified by a location signal including a location area identifier broadcast from a base station in that other location area to which the mobile station has moved.
Since the location of each mobile station is identified by location information which is the same for all mobile stations in a given location area, the location area also broadcasts a paging signal when a call is being sent to a mobile station present in that location area, and all base stations present in that location area are instructed to re-broadcast the paging signal even though the mobile station to be paged exists in only one base station area and not in the other base station areas.
For example, in FIG. 1, assume that an incoming call from the PSTN is being sent to a mobile station (not shown) in the base station area covered by base station 431 in location area 1. At this time, the MSC 42 accesses the database managed by the location register (not shown) to query the location information of the mobile station to which the call is being sent. The MSC 42 recognizes that the mobile station to be paged is present in location area 1 by the response of the database, and the MSC 42 then instructs all base stations in the location area 1, i.e., base stations 431 and 432, to broadcast the paging signal even though the mobile station to be paged is in only the base station area covered by base station 431. That is, regardless of which base station area the mobile station to be paged is in, the MSC 42 always instructs broadcasting of the paging signal to all base stations in the location area, which is the only information by which the MSC 42 can identify the location of the mobile station. Once the mobile station responds to the paging signal, the answer signal is received by the base station 431 and transferred to the MSC 42, and at this time, the MSC 42 can identify the particular base station area in which the mobile station exists.
In this conventional mobile station paging method, the number of paging signals broadcast into a location area is equal to the number of incoming calls to the mobile station within the location area, and is also equal to the number of paging signals broadcast by each base station in the location area. This is because the MSC 42 instructs all base stations in the location area to broadcast the paging signal. This means that the number of paging signals broadcast by a given base station is greater than the number of incoming calls to the mobile stations in the base station area which can be covered by the radio signal of any one base station, and also that each base station must broadcast paging signals for mobile stations not only in its own base station area but also in other base station areas in the same location area.
Thus, there is a problem that each base station has to broadcast additional paging signals for mobile stations not within its own base station area, and, as the number of base stations in a given location area increases, the number of signals to be transferred between the MSC 42 and each base station, and between each base station and the mobile station, is correspondingly increased. This in turn increases the signal processing load in the MSC 42 and in each base station, and also gives rise to the need for a transmission medium with high paging channel capacity.
Alternatively, it is possible to reduce the number of paging signals to be broadcast by reducing the number of base stations in a location area. In this case, however, there is a problem that the number of times of entering and leaving a given location area are increased because the location area is also reduced by reducing the number of base stations, so that the location information has to be updated frequently in the location register, and it causes an increased processing load at the location register.